A position, speed, and torque of a servo motor used in FA (Factory Automation) are controlled to follow a drive command (position command) from a host device (host controller), and digital control is broadly adopted, using a microprocessor as its control operation device. In a general PWM (Pulse Width Modulation) control system for controlling motor torque, there is a method of detecting and using a value of current flowing to a winding of the motor (hereafter referred to as “motor current”). In digital control of this method, a motor current value is periodically detected and the motor current is controlled to match a current command value, typically using PID control (proportional+integral+differential control). Torque output from a surface permanent magnet synchronous motor used in the servo motor is proportional to motor current, and thus the torque output from the motor can be freely controlled by matching the motor current value with the current command value by using the PWM control.
FIG. 8 is a configuration of conventional motor control device 90 including an inverter. In this conventional motor control device 90, current detecting resistance 91 for detecting a motor current value is provided between power converter 98, which is an inverter, and a winding of motor 30. AD (analog-digital) conversion unit 95 applies digital conversion to a voltage generated between both terminals of current detecting resistance 91 as the motor current flows, and supplies its digital data Di to digital controller 97. Conventionally, the motor current is generally detected with this configuration. Recently, however, the use of ΔΣ (delta sigma) AD converter 92, as shown in FIG. 8, in AD conversion unit 95 has been proposed with respect to less occurrence of gain error and offset (e.g., PTL1). This type of AD conversion unit 95 includes, for example, a photo coupler and digital filter, in addition to ΔΣ AD converter 92.
However, in the configuration of driving the motor using the PWM control, this ΔΣ AD converter is likely affected by leak current due to the PWM control.
More specifically, in the PWM control system, voltage applied to the motor is controlled by switching a switching element. Therefore, a leak current occurs at a moment of switching. Normally, the leak current flows to a grounded part typically through a casing and wiring. However, the leak current also flows via shunt resistance, and voltage at both ends of the shunt resistance changes by this leak current. The ΔΣ AD converter then converts this voltage to a 1-bit digital signal. Accordingly, a detected current value after an AD conversion decimating filter includes unwanted current component that is not originally flowing in the motor.
In the digital control, the unwanted current component is processed as disturbance, and voltage that cancels the disturbance is applied to the motor, causing undesired torque in the motor. In particular, at the time of servo lock and low-speed rotation in which the current flowing in the motor is small and switching timings of phases tend to overlap, an influence of the leak current becomes relatively large. Accordingly, a minute vibration of the motor output shaft occurs due to undesired torque even in the servo-lock state in which the motor output shaft should be still under normal conditions.